1. Field of the Invention
This invention relates in general to a lighting device of a discharge lamp, and an illumination apparatus using the lighting device.
2. Description of Related Art
In a conventional inverter lighting device, such as the one described in a disclosure in Japanese Laid Open H08-329731, film capacitors installed in electronic devices are subjected to thermal degradation due to heat generated by some electronic parts that will create heat in an operation. Ultimately, the lifetime of these film capacitors is shortened and their performance is changed. As a result, efforts are made to design a thermal shield to shield hybrid ICs or the layout of the circuit elements, so as to mitigate the thermal influence caused by the heat-creating parts.
Accordingly, a long-term use of a lighting device of a discharge lamp has to be considered. Therefore, a proper arrangement for the circuit parts must be examined so that the thermal influence can be reduced. Moreover, a film capacitor using a polyethylene terephthalate film having a high thermal resistance is also used.
However, for a capacitor using the polyethylene terephthalate film, the defective portions are removed by self-healing in an initial stage of dielectric breakdown. At the end of the lifetime of the capacitor, a short circuit or melting of the film is likely to occur, which becomes a problem for such capacitor. For capacitors made with a metallized film, self-healing or “clearing” can remove a fault or a short circuit in the dielectric film by vaporizing the metallization near the defect. The metallization is so thin that a negligible damage to the film is incurred during the clearing process. The vaporized metal oxidizes over time, aiding in the isolation of a fault area.
In order to solve the aforementioned problems, a capacitor with a polypropylene film is used. For example, the Japanese Laid Open Publication 10-119126 discloses a biaxial polypropylene film and a metallized polypropylene film to enhance the heat resistance and the properties formed by vapor deposition of these films by specifying an isotacticity and an isotactic pentad ratio. In addition, the Japanese Laid Open Publication 05-217799 discloses a vapor-deposited metallized film capacitor to obtain a vapor-deposited metallized film capacitor having excellent winding characteristics, excellent durability and heat resistance by providing a high rigidity vapor-deposited metallized film, formed by vapor-depositing a metal on a high rigidity polypropylene film. The Japanese Laid Open Publication 05-217799 discloses a polypropylene film capacitor to enhance long-term dielectric breakdown characteristics at high temperature and to suppress the deterioration in yield at a time of the production of a capacitor by specifying an isotacticity and an isotactic pendant ratio and a surface orientation coefficient of a biaxial polypropylene film. However, the polypropylene film will contract due to heat at the end of the capacitor's lifetime. The polypropylene film capacitor is easily affected by heat since the contraction temperature of the polypropylene film capacitor is lower than that of the polythylene terephthalate film capacitor.
Additionally, in order to mitigate the negative impact on the environment, a leadless technology is developed, i.e., no lead is included in the solder components of a circuit board. Materials having a high melting point are widely used as the solder components. As a result, a preheating temperature prior to the soldering process and a soldering dip temperature are increased, and the film capacitor is affected by the heat during a solder packaging process. The use of the polypropylene film becomes more difficult. Moreover, for a capacitor using a polypheylene sulfide (PPS) film that has a high thermal-resistant temperature, the PPS film does not have a high isolation voltage and its cost is very high.
FIG. 1 shows a temperature profile of a film capacitor during a soldering process and temperature increases at different positions of the film capacitor. FIG. 2 shows the positions on one electrode of the film capacitor where a thermal couple for measuring the temperature of the film capacitor is attached. In FIG. 1, the film capacitor uses a copper wire as its lead wires. In FIG. 2, a side view of the film capacitor 1 with one lead wire 2 is shown. The positions where the thermal couple is attached are labeled with a, b, and c. The temperature curves I, II and III in FIG. 1 respectively show the temperature profiles corresponding to the positions a, b and c.
In the experiment, the temperature profiles (curves I, I, III) in FIG. 1 show that after preheating at 120° C. for 90 seconds, the film capacitor 1 is lifted and then dipped in a soldering tank at 260° C. for 7 seconds. As shown in FIG. 1, when the copper wire is used as the lead wires 2, the temperature at the terminal end b of the film capacitor 1 rises up to about 150° C., in which the terminal end is referred to one end of the lead wire 2 that is attached to the film capacitor. In general, since the highest operating temperature (for a rated reduction use) of the polypropylene capacitor is 105° C., the polypropylene film of the film capacitor under 150° C. will have a large thermal contraction. Thus the capacitance of the film capacitor is reduced under this situation. If the film capacitor is kept in use, the capacitance of the film capacitor would be further reduced. This mechanism will be described below by using a metallized polypropylene film capacitor as an example.
FIG. 3A shows a perspective appearance of a film capacitor, and FIG. 3B is a perspective view showing an internal structure of the film capacitor. FIG. 4 is a cross-sectional view showing the internal structure of the film capacitor. Referring to FIGS. 3A, 3B and 4, the film capacitor 1 comprises two metallized films, metallic contacts 5 and lead wires 2, wherein each metallized film includes a film 3 and a metal layer 4 deposited on the film 3. Each film 3 is not fully deposited with the metal layer 4, and a portion of the film is exposed at one side. The two metallized films are stacked and then wound in a manner that the exposed portion of the films 3 are arranged in parallel at opposition direction, as shown in FIGS. 3B and 4.
Referring to FIG. 4, after the metallized films are wound, the metallic contacts 5 are formed by respectively spraying metal material onto two side faces of the wound metallized films and the metallic contacts 5 are thus served as electrodes of the film capacitor 1. Each metal layer 4 formed on the film 3 would be electrically connected to one metallized contact 5. The film 3 is a dielectric film and can use a polypropylene film which thermal contraction is large. Therefore, the metal layers 4 and the film 3 sandwiched therebetween form a capacitor. The two lead wires 2 are respectively coupled to the metallic contacts 5, through which the film capacitor can be mounted onto a circuit board. A resistance of the film capacitor 1 is a combination of a resistance of the lead wires 2 and the metallic contacts 5, a resistance of the metallic contacts 5 and the films and a resistance of the deposited metal layers 4, etc., which can be represented by a parameter tan δ. The parameter tan δ represents a dielectric dissipation factor, which is one of the capacitor properties. A dielectric dissipation factor is defined as cotangent of a phase angle or tangent of a loss angle for a dielectric material. Since the metallic contacts 5 are formed by spraying, the thickness of the metallic contacts 5 is not easily controlled to be uniform. Therefore, the resistance between the metallized contact 5 and the film 3 is easily unstable.
If the temperature during the soldering of the film capacitor 1 is high, the polypropylene film 3 will contract and an adhesion between the films 3 and the metallic contacts 5 will deteriorate further. As a result, the contact resistance will increase. In this situation, when a current is conducted to pass through the film capacitor, a portion of the film capacitor, where the contact resistance is increased, start generating heat, and the temperature is increased. As the temperature exceeds 105° C. for a long time, the thermal contraction of the polypropylene becomes large, and the adhesion between the films 3 and the metallic contacts 5 becomes worse. Therefore, the resistance of the film capacitor 1 is increased and the contraction is accelerated. Due to the contraction of the polypropylene film, an effective area of the polypropylene film of the film capacitor 1, which is sandwiched between two deposited metal layers 4, is reduced. Thus, the capacitance of the film capacitor is decreased.
In order to be cost effective and safe, the aforementioned polypropylene film capacitor can be used in a lighting device of a discharge lamp. However, a solution of how to reduce the effects of temperature and heat during the soldering process is a key issue.